Ultrahigh-frequency electron discharge tube apparatus



2,617,071 ULTRAHIGH-FREQUENCY ELECTRON DISCHARGE TUBE APPARATUS Filed Jan. 29, 1947 M. CHODOROW Nov 4, 1952 -2 smms-sx-msri T 5 INVENTOR M144 V/N Cf/ODORO w ATTORNEY Nov. 4, 1952 M. CHODOROW ULTRAHIGH-FREQUENCY ELECTRON DISCHARGE TUBE APPARATUS Filed Jan. 29, 1947 2 SHEETS-SHEET 2 Hill W INVENTOR M4,? V/A/ 67/000901 ATTORNEY Patented Nov. 4, 1952 ULTRAHIGH-FREQUENCY ELECTRON DISCHARGE TUBE APPARATUS Marvin Chodorow, Kew Gardens, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application January 29, 1947, Serial No. 725,106

6 Claims. 1

The present invention relates to the art of ultra-high-frequency apparatus, including electron discharge tubes of the velocity modulation type, and utilizing cavity resonators operating at frequencies of the order of 1,000 to- 30,000 megacycles per second. More particularly, the invention relates to arrangements for coupling said tubes to other high-frequency apparatus and for varying the operating frequency of the tubes.

Cavity resonators, as employed in high-frequency electron discharge tubes or other highfrequency apparatus, have been made in a variety of symmetrical shapes and with various types of coupling devices for introducing highfrequency energy into or extracting high-frequency energy from the resonators. The operating frequencies of such resonators have been variably controlled by diverse types of tuners, of which one known form comprises a conductive rod member mounted to be adjustably positionable within the resonator so as to alter the value of frequency-controlling parameters thereof and to change the frequency of operation of the tube accordingly.

However, the tuners and couplings heretofore employed have been selected in the light of mutually independent and, generally, incompatible design and operational requirements. Thus, in the prior art, conductive rod tuners have been introduced into regions of intense electricfield or intense magnetic-field components of an excited cavity resonator, thereby causing alteration of the equivalent capacitance or the equivalent inductance of the resonator. The former method is generally referred to as capacitive tuning of the tube while the latter is known as inductive tuning.

Where such capacitive or inductive tuning means are embodied in conventional symmetrically shaped cavity resonators having conventional input or output coupling loops, it has been observed that such a combination of tuner and coupling is seriously limited as to operable tuning range because of' the diificulties experienced in orienting the conductive rod so as to lie entirely in either the electric-field portion or the magnetic-field portion of the electromagnetic field within the resonator. It has been found, for example, that insertion of the tuning rod beyond a certain limited amount fails to provide frequency alteration corresponding to the increased insertion, due to the fact that the rod then lies ina region occupiedby bothelectricand magnetic-field components of substan- 2 tially equal magnitude, thus producing practically nonet tuning effect. As a result of such tuning-range limitations, the utility of the cavity resonator apparatus of the prior art has been seriously restricted.

A principal objecttherefore, of the present invention is to provide generally improved tunable cavity resonator apparatus wherein the above-described difficulties are avoided.

Another object is to provide a high-frequency electron discharge tube of the velocity-modulation type having an improved cavity resonator embodying novel arrangements of tuning and coupling means mutually cooperable to produce a substantially increased range of tuning for the tube.

In accordance with the present invention, broad-band tuning is accomplishedin an ultrahigh-frequency electron discharge tube device by means of a novel type of cavity resonator having an axially slotted wall portion and wherein a waveguide coupling arrangement and a conductive tuning member are provided in substantially diametrally opposed portions of the resonator. Increased range of tuning has been found to result from the fact that the asymmetry of the resonator structure produced by this novel arrangement enables the positioning of the tuning member substantially exclusively in the magnetic-field portionof the resonator field thereby minimizing: undesired capacitance effects. Furthermore, it has been discovered that variations of the elfective inductance value of the resonator portion occupied by the tuning member, in accordance with the present inven-.- tion, produces a relatively greater change in the total effective inductance'oi the resonator than is produced-by a corresponding change in conventional resonators.

Accordingly, it is another object of the present invention to provide an improved high-frequency apparatus having incre'a's'edtuning range provided by an asymmetrical resonator arrangement including an axially slotted cavity resonator having a wave guide coupling at the slotted portion of the resonator andaconductive tuning member in a .diametrally opposed portion of the resonator.

Still another object of the invention is to provide a cavity resonator device of increased tuning range comprising anaxially slotted cavity resonator having a, wave guide section coupled to the slotted portion thereof and a conductive tuning member adjustably disposed within a portion of the resonator diametrally opposed 3 from said slotted portion and translatable chord wise therein, whereby, for a certain mode of excitation of said resonator, adjustment of the position of the tuning member produces substantially pure inductive tuning of the resonator.

A feature of the present invention lies in the provision of an ultra-high-frequency apparatus comprising a cylindrical-cavity resonator having an axially slotted wall portion, a wave guide section coupled to said resonator at the slotted portion thereof, and a tuning rod adjustably positionable within the resonator in a region diametrally opposed from the slotted wall portion.

Another feature of the present invention is the provision of a novel reflex oscillator of the klystron type having a waveguide coupling arrangement and a conductive: rod tuner cooperable with said coupling arrangement to produce an increased range of tunability for said oscillator.

The invention in another of its aspects relates to novel features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used for the said principal objects or in the said field.

A further object of the invention is to provide improved apparatus and instrumentalities embodying novel features and principles, adapted for use in realizing the above objects and also adapted for use in other fields.

The above and other objects and advantages of the invention will be apparent from the particular description thereof made in connection with the accompanying drawing in which:

Fig. 1 is a plan view, partly broken away, of a conventional type of coaxial cylindrical cavity resonator usefulin describing certain features of the present invention,

Fig. 2 is a schematic showing of an equivalent lumped-constant circuit of the cavity resonator shown in 1,

Fig. 3 is a transverse, cross-sectionalview of a cavity resonator embodying certain features of the present invention, Fig. 4 is an elevational view, partly in longitudinal section, of an ultra-high-frequency velocity-modulation reflex oscillator embodying a resonator of the type shown in Fig. 3,

Fig 5 is a cross-sectional view of the oscillator of Fig. 4 taken along the line 5-5 thereof, and

Fig. 6 is a fragmentary cross-sectional view taken along the line 65 of Fig. 4.

Referring to Fig. 1, a conventional type of ultrahigh frequency cavity resonator is shown having a conductive cylindrical wall H, a coaxially disposed conductive inner rod l3 and end walls [5 and H enclosing the cylinder at top and bottom, respectively. Rod I3 is connected at one end to one end wall, such as 15 and is adjacent but spaced from the other end wall. As is well-known, the enclosure thus formed defines a space resonant system, which, for a given mode of oscillation, is resonant ata frequency, J, defined by 4 lumped inductances of inductors I9, 2! respectively represent the equivalent inductances L1, L2 of the respective half-portions 25, 21 of the resonator (Fig. l) to the left and right of a diametral plane 29 drawn through the resonator, while the lumped capacitance of capacitor 23 represents the equivalent capacitance of the resonator.

The joint equivalent inductance L of the resonator is approximately given by the well-known relation Since the resonator is symmetrical about the plane 29, the equivalent inductance L1 is equal to the equivalent inductance L2, and for this condition Lz L1:1/2 L2.

For a fixed value of the equivalent capacitance C, the resonant frequency f of the resonator depends on the value of the joint equivalent inductance L. In the conventional cavity resonator case disclosed, the equivalent inductances L1, and L2 of half-portions 25, 21, respectively contribute equally to the joint value of the equivalent inductance L. A limited degree of variation of the inductances of either of the half-portions 25, 21 is provided, in the prior art, by inserting a conductive rod 3i, or similar conductive member, radially into the resonator. The variation of inductance thus produced gives rise to a corresponding variation of the resonant frequency of the resonator. As noted hereinabove, such frequency variations have proved to be seriously limited as to workable range.

By the present invention, a substantial increase in the range of frequencies over which such a resonator may be tuned is provided by the use of the cavity resonator illustrated in Fig. 3. Here, the resonator is generally similar to the resonator H, l3 of Fig. 1, with the exception that a portion of the wall 33 is cut away as by a plane passed parallel to the axis of the cylinder, and at a predetermined distance therefrom, to form a longitudinally extending slot 35 the Width X of which depends on the said predetermined distance from the axis at which the slot is formed.

The equivalent inductance of the region adjacent the slotted wall section of resonator is larger than that of the unslotted section; that is, the region remote from the slot, by an amount proportional to the width X of the slot 35. Thus, since the slot 35 may be made of any desired width consistent with mechanical design considerations, the inductance L2 may be made correspondingly large. For a slot width X of order of magnitude approximately one-third the diameter of the cylinder, or greater, the total equivalent inductance L of the slotted resonator is largely determined by the inductance L1 of the unslotted portion. It will thus be seen that under such conditions variation of the inductance L1 of the unslotted portion produced, for example, by variation in position therein of a conductive tuning rod 31, gives rise to a greater proportionate effect on the total equivalent inductance L, resulting in a correspondingly greater effect on the resonant frequency of the resonator.

An alternative way of considering the increased tuning range of a cavity resonator having a slotted side wall portion is to regard the resonator as a coaxial line section terminated in a capacitance. If the characteristic impedance of the coaxial line section be Z0, the effect of introducing a conductive rod into the space between the inner and outer conductors of the line is to stirrer impedance with respect to eccentricity,

re de increases as the value of the eccentricity is in creased. Thus, a given displacementcf the "outer conductor "of the line ofi concentricit y will have a greater efiect on the value of the char acteristic impedance, the greater the valued the eccentricity. Therefore, it follows that the ing range of a plunger employed in such anew paratus will be increased if the line is made initially eccentric as by providing a slot in the side wall of the outer conductoi While alternative explanations of the principles on which the novel features of the invem tion may be based have been proffered, it is to be understood that the invention is not to be conditioned or limited upon either or both of these explanations, these having been gi'ven'merepermitting a greater volume of resonator to be occupied by the rod, than is possible by the .prior art radial motion. v

Figs. 4 to 6 show an adaptation of my broadband tunable cavity resonator to an ultra-high frequency electron discharge tube which, 'in the illustrated embodiment, is of the electron v'elocity-modulation type, known in the 'art as a reflex klystron,

It will be understood that the invention is not limited to application in reflex lglystrons, but may be utilized in all types of high irequenoyapparatus wherein broad-Fband tuning is desired.

As shown, tube -39 comprisesa eavityresonator 4| formed by a substantially cylindrical conclu e tive wall 43 and a pair of centrally apert rred cone c iv e d wa s 5. It will be not d that s na o i prefera ly fo me w th re ative 1y th c m t l c we s icr ccd eet=ra iat n chara te t c th r y mi im ng ndesirable fr qu ncy dr t n due t a teret e c e rear nator dimensions caused by flu crat n r ambient t mp a ure 5 It will be noted that, while thefwalls it! are herein disclosed as being for ed;1'rem solid block of conductive material 'su ably ina ch m d to d fine thcjc v ty resen, r closure is merely illustrative o *oneni'an fabrication. Such a resonat r may. if be formed in diverseother W H1 ing and interconnecting suitably 'foi'medfsheets or laminations of metal M 1011, 1' a 2111 5 3, fine substantially the same 'ressnstoreqnfigum. tion as disclosed. n

Supported in the "central aper "L175 ofend "W311 45 and projecting coaxiall'y 1111 r o a is a tubular member 49 having a h -id 138 1 91 grid structures 51, 53 at its inn and-o 6]: ends respectively. The aperture 'of end wall! may-be provided with a similar grid'sti'ucture We which, together with grids 5|, 53 amorous-n den-he a path for a beam of electrons supplied by a catm ode assembly 51 or any suitable eonventiomi type. cemeae assembly 51 may inciuc'ie a focus in": control or 'incdulastfiig electrodes. as desired.

For reflecting the beam of electrons, a reflector electrode '59 is rigidl sealed into'the tube 39 by means or a structural arrangement which i's'par denier-1y adapted to iii i-nimi-ze 'microphbnics, This arrangement compris s a conductive tutu-- lar member *61, vacuum sealed to the outer sur fa'ceof end wall 4'1 and "coaxial alignment with the sent-rel aperture therein. outer end-6r Y tubular member s1 is fiarea as at 53 to prsviee EL1511611 1361" at Whibh '3, correspondingly fla'itd outer periphery of a eerieav iy dished cap -65 may be sealed to for-in an air-tight, ir'lbhafiieally rig d joint.

Gap 6 5 centrally *apertured for receiving an insulating sup ort 61 which is formed with a dwhwa'rdly X-tefidi fig' hollow Gylilifiiibhl part 5'9 and 2. bush 1 L The S11iiiioit may be of glass bi other electrical insulating material suitable for bonding to metal. Bush H is preferably vacuumsealed into the aperture of cap '55 with the free end of the cylindrical part 59 bonded in butt fashion to "reflector -1i3tr0de 59. Lead-fin Wife 13 'for providing electrical connection between reflector electrode 59 and an external -s'our'ce of a potential, not shown, is suitably molded the insulating "support 61 and conductively -'connected to reflector "electrode 59.

thus far fit-isii-r ibed, fliedeflex :k'ly's'tr'lin tube 39 is Of substantial-1y conventidnm design aha-O'D- eration. The electrons from the cathode 51 are accelerated to form a beain by a suitable voitage impressed between grid 53 and Cathode 51. beam 'in'iti'a'lly traverses the gap between i'eso nator grass '51, '55 and is thereafter returned into the resonator 4| by reflector electrode 59 s as t0 eiiiiit and maintain an oscillatory electromagnetic 'fie'ld therein. Accepted theories or this a'iifiibh based on Velocity modulation principles are xpiamed fully n Fig.2 or U. si s-tent No. 2,250,511, i'ss'iiil July -29, 1 94 1, to Russell H. Vari'an and William W. 'I-Iansen, to which reference is made for fiirtheriietails.

fWi'd'e-band inductive tunih'gj'of tube 39 is as eomplished'by 'm'ean'sof a movable plunger which may be in the term of a conductive rod 15 mounted for chordw'ise projection into the inti idiof resbfiatoi I. AS-Shawl}, T03 1515 Dre fer'ably cylindrical -i"n shfape'and or-eros'scsectienal dimension radically to fill the space between end "walls '45 hid '41 "and tubular member an?! the ad acentp A oh 'o f 'cyiindricalwal1 4'3. iA ISO, the nner end'o'f rod 151s er ra yrounded is form "a hemispherical tip to reduce undesired capacitance 'efieet's.

Variable adii'istifi-iit' 'df"=l0t1 15 from the blitside tube 39 is aflo rde'd through the use of a vacuum-tight bellows arrangement to "be described l "hows the' s'tructure "of one type of bl lows arrangement which "may conveniently be uses to adjust the position or obiiductixierbd 15. his 'disemsed, the rrangem nt is fixedly attached to the outside-of the cyll'ilfifioal will 4'3 of reson'ator ll at "a chordw'ise extending opening 11 formed in "the war. Opening 11 communicates with 'the interior of the "resonator and is "formed with a pal'rof coa'xiallyfiisposed, relatively smallbore and large-bore portions '19, "81. respectively.

"thr ugh Whichrbd '15 "(extends A sleeve 83 having an axial bore of substantiarlly the "same "diameter as that of the smallbore portion 19 and 'a radial 'flange 85 at the inner 'end; is -rigidly seated the iarge bere por tion SI of the opening with the-flange 85 brazedor otherwise conductively connected'to the res-.

onator wall 43. A second sleeve 81, having an annular shoulder 89 formed atits inner end and a radially extending flange 9I at the other end, is tightly fitted and conductivelysecured in the large-bore portion 8| of opening 11, with sleeve 83 telescopically received within sleeve 81. The outer diameter of sleeve 83 and the diameter of the bore of sleeve 81 are so selected that, when assembled, the adjacent lateral surfaces of the sleeves are slightly spaced, forming a cylindrical slot or space 93 extending coaxially between sleeves 8 3 and 81. The bore of sleeve 81 is slightly longer than sleeve 83 so as to provide a radial slot or space 95 which, as shown, connects with the cylindrical slot 33 to form a continuous inverted U-shaped slot in cross-section, the overall length of which is chosen. to be substantially one-quarter wavelength at the average operating frequency of the tube.

As is well-known inthe art, a slot of the inverted U-shaped type hereinabove described forms a folded short-circuited quarter-wavelength transmission line section which provides a-very low impedance, amounting practically to a short circuit for ultra-high-frequency energy of frequency corresponding to the average operating frequency of the device, as seen looking along opening 11 from the interior of the resonator 4I outwardly. Accordingly, it will be seen that practically no electromagnetic energy of this frequency can leak out of or be radiated from the interior of the resonator through the opening 11. .Other suitable means may be provided for the same radiationinhibiting or wave-trap purpose. 1 Theradially-extending flange 9| of sleeve 81 provides an anchor plate for mounting one end of a-flexible bellows 91 invacuum-tight manner, the other end of which is fixedly secured and sealed to a radially extending flange 9 9 of an internally threaded travelling nut II which is adjustable toward or away from flange ill by rotationof arotatable screw shaft H13 in one sense or the other. Rod 15 may be aflixed to the nut Ifl-I .in any suitable manner or may be integrally formed with nut IN to be movable therewith, and thus to be .adjustably translatable within the resonator II. As shown,,the outer end of shaft I03 is turned smooth so as to extend through and be rotatable in a bearing I 95 provided in a-metallic-cylindricaldust-proof cover II3 which, in turn, is; preferably detachably connected to flange 9I,-as by means of machine screws II5. Shaft I93 is secured; against inadvertent translatory movement by means of a snap-washer I fitted into an annular groove in the shaft I03 and a similar washer, not shown; at the other end of bearing I05. A knurled knob I01 may be provided to facilitate manual operatlon of the screw shaft I03. Also, a pointer I09 and scale I II may be provided if visual indication of the position of the rod and hence of the resonator frequency. is desired. For-coupling tube 39 to a load, which, in the illustrative embodiment is represented as a wave guide section H1, a slot H9 is formed in. a side wall portion of the resonator Al on the side substantially diametrally oppositethe tube 49. Such a slot may be formed by milling or otherwise machining a groove I2I, Fig. 4, into the wall 43 ,of the resonator to a depth, corresponding to a desired width of slot. It will be understood that, W r th sqn t risbu l p b t ckinane formed; metallic, laminations, as hereinabove where the, resonator is built up by stacking preliminarily forming such laminations so as to provide, in assembly, such agroove and slot.

A section of oval wave guide I23, dimensioned above cut-off at the mean operating frequency and having spaced parallel top and bottom walls I25, I21 and substantially cylindrical side walls I29, I3I, Fig, 6, ismounted in the groove I2I at the slot I I9; One end of the wave guide I23 is open and communicates with the interior of the resonator 4| through slot II9. As shown in Fig. 6, the transverse dimensions a, b of wave guide I 23 are preferably substantially greater than the dimensions of slot I I9. The other end of the wave guide-is vacuum sealedas at I33 by means of any suitable dielectric material which is readily transparent to high-frequency electromagnetic waves.

Wave guide section I23 is adapted to be telescopically received within the wave guide I I1, as shown, and secured thereto by means of machine screws I35 threaded into suitable openings in a conventional choke flange coupling I31 having one portion fixed to resonator 43 and a second cooperating portion fixed to wave guide II1. It will be understood that any suitable wave trap I 39 or radiation-inhibiting coupling arrangement between the wave guide sections I I1 and I2I may be utilized as desired. It has been found that the wave guide coupling herein described affords a wide selection of degrees of coupling between the electromagnetic energy within the cavity resonator of tube 39 and an external load, the factors determining the degree of coupling being the width :2: of the slot H9, the length Z of the wave guide I23 extending into wave guide H1 and the transverse dimensions d, b of wave guide section I23. It has also been found that, by selection of suitable values for these "factors, substantially any desired degree of coupling may be obtained.

In one embodiment, for operation in the band 'of frequencies ranging between 4800 megacycles per second and 5200 megacy-cles per second, satisfactory operation into a load in the form of conventional l-inch by 2-inch wave guide is obtained by means of a wave guide section I2I having the following valuesi zc= inch 1=1% inch 'a.=1.8 inch b=0.5 inch ,Itwill, of course,v be understood that where other degrees of coupling are desired, or where operation ata different range of frequency values is desired, othersuitable values for these factors may be selected. A Ithas also been found that in the above-described embodiment, utilizing the values indicated, a range of variation of operating frequency of. 400-600 megacycles per second is obtainable, which represents aconsiderable increase over that afforded in prior-known types of apparatus. Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

cavity resonator as a frequency-determining element thereof, said resonator having a slotted portion, the region of said cavity resonator adjacent the slotted portion thereof having a substantially larger effective inductance compared to the effective inductance of said cavity resonator remote from said slotted portion to render said cavity resonator asymmetrical, said resonator being suitable for supporting an electromagnetic field therein, a conductive tuning member projecting into said resonator through a cylindrical wall portion thereof substantially diametrally opposite said slotted portion of said resonator, said tuning member being movable substantially entirely within a region of said resonator suitable for containing magnetic field components of said electromagnetic field, and means coupled to said tuning member for varying the amount of projection of said tuning member within said cavity resonator so as to vary the eifective volume thereof, thereby producing broad-band variation of the operating frequency of said device.

2. Ultr'a-high-frequency electron discharge tube apparatus comprising means for forming and projecting a beam of electrons along a predetermined path, conducting walls defining a hollow cavity resonator surrounding the path of said beam, said resonator having a slotted wall portion, a first portion of said resonator having a larger effective reactance value than a further portion of said resonator, means in the path of said beam for reversing and reprojecting said beam through said resonator, whereby ultra-high-frequency energy is generated Within said resonator, and tuning means in a further portion of said resonator for varying the effective reactance value of said further portion, said tuning means including a conductive member adjustably disposable in said further portion, whereby the difference in the effective reactances of said portions produces upon adjustment of said conductive member a relatively large variation of the total effective reactance of said resonator.

3. High frequency apparatus comprising conducting walls defining a cavity resonator, said cavity resonator having an opening in a wall portion thereof, a first section of hollow wave guide sealed at one end thereof at said wall portion about said opening, a gaseous-tight seal at the other end of said wave guide whereby said first section of hollow wave guide together with said conducting walls defining said cavity resonator comprise a unitary gaseous-tight body, and a secondsection of hollow wave guide receiving said other end of said first section of wave guide, said first section of wave guide projecting a predetermined electrical length into said second section of wave guide and having predetermined cross-sectional dimensions to effect a desired impedance discontinuity so that a desired coupling sensitiveness for energy coupled from said resonator to said second wave guide is aiforded.

4. A tunable cavity resonator comprising a conductive substantially cylindrical wall means having a slot, spaced conductive end wall means disposed at the ends of said cylindrical wall means, an electron-permeable gap centrally disposed with respect to said cylindrical wall means, the effective inductance of a first region of said cavity resonator adjacent said slot being relatively large compared to the effective inductance of a second region of said cavity resonator, and a conductive member having chordwise translation within said cavity resonator for varying the effective volume thereof, the axis of said conductive member being displaced from said electron-permeable gap, said conductive member being adjustably disposable within said second region and being further substantially diametrally opposite said slot to produce a variation of the effective inductance of said second region, whereby the operating frequency of said resonator may be varied over a relatively wide range. v

5. A tunable cavity resonator comprising a conducting body having substantially toroidal inner wall portions, said cavity resonator being inductively asymmetrical about its axis of revolution, one region of said resonator having a relatively large effective inductance compared to a second region thereof, said resonator having an aperture in the wall thereof adjacent said one region, and means for varying the effective volume of said cavity resonator, said last-named means comprising an adjustable conductive element confined to said second region within said cavity resonator beyond said axis of revolution and remote from said aperture for producing a variation of the effective inductance of said second region, whereby the operating frequency of said cavity resonator may be varied over a relatively wide range.

6. Ultra-high frequency electron discharge tube apparatus comprising a cylindrical cavity resonator having hollow axial reentrant portions defining an electron permeable gap, said cavity resonator having an aperture through the cylindrical side wall thereof, waveguide output means including a gas-tight seal coupled to said cavity resonator for receiving ultra-highfrequency energy through the aperture in said side wall, and a conductive tuning plunger projecting into said cavity resonator through the cylindrical side wall at a region separate from said aperture to vary the volume of said cavity.

resonator, said tuning plunger being movable along a chord of said cavity resonator lying between said gap and said side wall of said cavity resonator whereby the plunger extends into said cavity resonator for a distance greater than the radial distance between said gap and said side wall.

MARVIN CHODOROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,106,769 Southworth Feb. 1, 1938 2,129,712 Southworth Sept. 13, 1938 2,200,023 Dallenback May 7, 1940 2,259,690 Hansen Oct. 21, 1941 2,403,025 Samuel July 2, 1946 2,423,383 Hershberger July 1, 1947 2,424,089 Gethmann July 15, 1947 2,426,177 Carlson et al. Aug. 26, 1947 2,426,193 Fernsler Aug. 26, 1947 2,427,106 Landon Sept. 9, 1947 2,438,768 Stewart Mar. 30, 1948 2,451,825 Guarrera Oct. 19, 1948 2,491,418 Schlesman Dec. 13, 1949 2,496,772 Bradley Feb. 7, 1950 FOREIGN PATENTS Number Country Date 579,648 Great Britain Aug. 12, 1946 

